The isolation of the cystic fibrosis conductance regulator (CFTR) gene encoding the defective protein in cystic fibrosis (CF) has led to the development of strategies of using somatic gene therapy to treat this disease. Because much of the mortality associated with CF is related to pathology in the lung, efforts to deliver replacement gene therapy directly to airway epithelium have been undertaken. CFTR gene therapy via replication defective adeno and retroviruses has been demonstrated in mature airways. Because the mature resting airway epithelium proliferates slowly, incorporation of targeted genes into the progenitor cell population necessary for permanent complementation may not be possible. In the early developing lung, however, the airway epithelium is highly proliferative and relatively undifferentiated. Fetal epithelial cells targeted by viral vectors containing the CFTR gene could result in the complementation of airway progenitor cells, and thus lead to long-lasting therapeutic benefit. This pilot application describes three types of studies aimed at evaluating the feasibility of gene therapy of fetal lung for the treatment of CF. First, because explanted fetal lung tissue continues to undergo near normal airway in vitro for prolonged periods of time, the conditions for optimal gene therapy of the explanted lung from the mouse using retroviral and adenoviral gene targeting vectors expressing the lacZ and CFTR proteins will be evaluated. Second, functional complementation of the transferred CFTR gene will be assessed, by comparing treated and untreated explants from normal and CFTR deficient embryos for transepithelial voltage technique for gene transfer to human fetal lung will be tested using explants from normal human fetuses. The studies proposed in this application should provide the necessary information to determine whether fetal gene therapy in the treatment of CF is possible and efficacious.